Numerous applications exist for Opto-Electronic Integrated Circuits (OEIC's) that can utilize a single semiconductor structure, including monolithically integrated waveguide lasers, amplifiers, modulators, splitters, phase shifters and so on.
Quantum well structures are used in optical waveguides and have also been disclosed in a number of publications, including one by T. H. Wood entitled "Multiple Quantum Well Waveguide Modulators", Journal of Lightwave Technology, vol. 6, pp. 743-757, June 1988.
However there are a number of problems, disadvantages and shortcomings with the integration of current optical waveguide structures due to different bandgap requirements for different devices. Among the shortcomings in this area are problems caused by epitaxial regrowth, the difficulties of wafer to wafer interconnection and problems associated with selective growth. Until now, prior art waveguides and similar related structures have by necessity been provided on two or more physically discrete semiconductor material structures. This requires taking the semiconductor materials on and off of the crystal growth chamber several times and also necessitates the costly and time consuming process of then matching these semiconductor materials with one another and then attaching them to each other. The devices of the present invention solve those optical waveguide structures problems, disadvantages and shortcomings by advantageously tilting a valence band quantum well double heterostructure.
The devices of the present invention provide novel opto-electronic integrated circuits having a unique tilted valence band quantum well heterostructure. This permits the manufacturer to employ a semiconductor heterostructure with only one growth that avoids the long-standing difficulties of epitaxial regrowth and wafer to wafer interconnection as well as the numerous problems associated with selective growth.
Active semiconductor waveguides are those with gain. It is well known that active and passive semiconductor waveguides operating in the same wavelength have different energy bandgaps. The devices of the present invention provide a novel material structure based on tilting the quantum well in an advantageous manner that provides both the active and passive waveguide sections on the same waveguide material. These devices function simply by their normal operating in a forward voltage bias for active waveguides such as lasers and amplifiers and operating in a reverse voltage bias or 0 bias for modulators or passive waveguides. The present invention uses the same physical structure for either bias orientation but applying a reverse bias allows it to function as a different device, i.e. an amplifier can function only as an amplifier when forward bias is applied. By allowing operating both in the forward and reverse directions on the same material, the tilted quantum well heterostructure devices of the present invention eliminate complex and expensive fabrication processes such as regrowth and interconnection. Moreover, such a tilted quantum well double heterostructural arrangement can markedly improve the quality and efficiency of the device, making it ideal for OEIC's.
Using both the graded composition and different conduction band offset ratios between the quantum well and barrier interfaces causes a large electrical field for the heavy hole valence band in the quantum well, which is the opposite of the well-known p-i-n field of standard waveguide device structures. The advantage of this invention's new arrangement is that the absorption edge, or transition energy, of the waveguide structure can now be widely tuned to lower energy with a forward bias, and then tuned to higher energy with a reverse bias, due to the quantum confined Stark effect described in D. A. B. Miller's "Electric Field Dependence of Optical Absorption Near the Band Gap of Quantum-Well Structures," Physical Review B, vol. 32, pp. 1043-60, July 1985. Based on the advantages of the present invention, both active and passive waveguide devices can now be fabricated on the same structure by applying their normal operating biases. This can be done without the cumbersome, complex and costly additional fabrication procedures previously required for fabricating such waveguides with two or more physically discrete semiconductor material structures.
Another prior art publication considered relevant in addition to T. H. Wood's "Multiple Quantum Well Waveguide Modulators" and D. A. B. Miller's et al., "Electric Field Dependence of Optical Absorption Near the Band Gap of Quantum-Well Structures", cited above, is D. A. B. Miller et al., "Band-Edge Electroabsorption in Quantum Well Structures: The Quantum-Confined Stark Effect," 53 Physics Review Letters 2173 (1989).